A T-shirt vendor saw something suspicious in New York's Times Square Saturday evening -- smoke coming from an unoccupied SUV on 45th Street near 7th Avenue -- so he alerted police.
Authorities found potentially deadly explosives in SUV left in Times Square.

The tip led to what New York Mayor Michael Bloomberg called an "amateurish"-looking car bomb and what Secretary of Homeland Security Janet Napolitano told ABC News' "This Week" today might have been part of a "one-off" attack.

The 6:30 p.m. scare prompted police to evacuate thousands of people from the heart of the Big Apple during one of its busiest times -- a warm Saturday night when it was packed with theatergoers and tourists.

"We are very lucky," Bloomberg said. "Thanks to alert New Yorkers and professional police officers, we avoided what could [have been] a very deadly event."
Authorities were examining security cameras and other evidence to see if they could identify a possible suspect or motive -- and already had located video of the car being driven to the scene.

"Tape is being reviewed and additional forensics are being done in addition to that," Napolitano said. "Times Square, I think, now is safe."

President Obama was being kept informed on the investigation by Deputy National Security Advisor John Brennan, who was communicating with the New York Police Department and other investigators, the White House announced Saturday evening.